Thermodynamic parameters of the helix–coil transition in polypeptide chains. III. Random copolymers of L‐leucine with L‐glutamic acid

Abstract

SynopsisConformational transitions induced by pH changes in random copolymers of leucine and glutamic acid have been studied. Significant differences were observed in the potentiometric titration curves of copolymers with small (up to 4%) and large leucine contents. The helical stability of copolymers with small leucine content, determined from titration curves by the Zimm and Rice method, decreases slightly with an increase in the leucine content, whereas the helical stability of copolymers with large leucine content increases sharply with an increase of the leucine content. It is shown that copolymers with large leucine content aggregate in the region of transition into the helical state, but the increase of their helical state stability is not connected with intermolecular aggregation, as it was also observed for a nonaggregating fraction isolated from one of the copolymers by gel chromatography. A conclusion is made that the helix-coil equilibrium constants for leucine does not itself exceed the s constant for uncharged polyglutamic acid. The stabilization of the helical state in copolymers with large leucine content is due to intramolecular aggregation of helices in these copolymers. The analysis of the leucine residue distribution between helical and nonhelical regions in globular proteins also gives no real arguments to ascribe special helix-forming properties to leucine.

A synthetic polypeptide with a compact structure and its self‐organization

“…Anomalously high PKef f values at ct < 0.5 (i.e. anomalously low values of the effective ionization constant) are evidence of a great electrostatic interaction of Glu residues, which suggests the presence of a compact structure of this copolymer at acid pH [2].…”

“…Repeated gel-chromatography of this fraction at pH 5.2 gave a fraction which does not aggregate even at this pH and contains 17% Leu (see [21 ). Tbis fraction was studied by potentiometric titration, optical activity dispersion (see [2]) and polarized luminescence. For comparison a study was also made of polyglutamic acid (PGA) and a Glu + Leu copolymer containing 4% Leu.…”

“…It has been shown in a previous paper [2] that a nonaggregating fraction can be isolated from random copolymers of Glu and Leu. The potentiometric titration curve of this fraction allowed one to presume the existence of a compact structure in a weakly ionized state.…”

Polarized luminescence and mobility of tryptophan residues in polypeptide chains

“…Then the fraction nonaggregating at pH 4.8 was isolated from the triple copolymer of glutamic acid with leucine and tryptophan (Glu, Leu, Trp) as described in Ref. 12.…”

Self-assembly and pH-responsive properties of poly(L-glutamic acid-r-L-leucine) and poly(L-glutamic acid-r-L-leucine)-b-polysarcosine

“…Poly-L-leucine (polyLeu) is insoluble in water in any pH and most organic solvents [16] . Some properties of random copolymers of L-glutamic acid with L-leucine [poly(Glu-r-Leu)s] have been reported including helixcoil transition [17,18] , adsorption dynamics at an oil/water interface [19] , and surface interactions with cellules [2022] . To the best of our knowledge, pH-dependent phase-transition of poly(Glu-r-Leu) has not been studied yet.…”